1. Field of the Invention
The present invention relates to a shell structure and, more particularly, to an inner shell preferably used for integrally forming the seat with the seat back of a chair having a frame construction in which the seat and seat back move synchronously.
2. Description of the Related Art
There have hitherto been proposed various types of office swivel chairs in which the seats and seat backs thereof move synchronously. More specifically, members for attaching and supporting the seats and seat backs are rocked, whereby the seats are moved up and down, and the seat backs are moved back and forth.
There are two types of rocking mechanisms: a rocking mechanism in which the seat portion and the backrest of a chair are rocked on the same component because only one member is used for attaching the seat portion and the backrest; a rocking mechanism in which the seat portion and the backrest of a chair are rocked on two or more different components because the seat portion is connected through a link member to the backrest. In both cases, from the viewpoint of human engineering or the functions of the seat and backrest, the seat portion and the backrest are designed so as not to have the same range in which they are rocked.
A range in which the seat portion moves differs from that in which the backrest moves. When these two components are supported by an inner shell, in which both components are integrally formed, or by such an inner shell and an outer shell, the inner or outer shell must have elasticity and a construction so that either shell can move as various components of a chair move in a manner different from that of either shell, even though the seat portion and the backrest are made of the same material.
On the other hand, many chairs, each having a shell structure which permits a backrest thereof to incline, do not incline smoothly. This is because the repelling force of a repelling member, such as a spring, is too strong, or because friction occurs between an inclining shaft of an inclining mechanism and a bearing thereof.
The use of, for example, a spring having a weak repelling force enables the backrest of the shell structure to incline rather smoothly. However, even when only a little load is applied to the backrest, as when a seated person changes the position of his back, instead of a heavy load being applied to the backrest, as when the person leans against the backrest, the backrest inclines unstably and too readily.